Geochemistry And Petrogenesis Of The Postcollisional Potassic-ultrapotassic Rocks In Tibetan Plateau

During the postcollisional tectonic-magmatic evolution stage of southern Tibet,the Oligocene-Miocene potassic-ultrapotassic volcanic rocks and ore-bearing adakitic porphyry occurred in an east-west extensional setting in the Lhasa terrane.As the magmatic products of geodynamic processes at depths,postcollisional magmatism in southern Tibet offer a unique opportunity to investigate the geodynamic processes responsible for plateau uplift andto probe into the nature of mantle lithosphere,which thus provide critical constraints on discussing the sediment subduction and recycling during the obliteration of the Neo-Tethyan oceanic slab and the continental convergence between India and Asia.However,many important issues including the petrogenesis of mantle-derived ultrapotassic rocks,the nature ofmantle sources,and the related geodynamic processes remain in dispute.Meanwhile,potassic volcanic rockswere treated as a kind of adakitebecause of their geochemical features.But this explanation ignores the petrogenetic differencesbetween potassic volcanic rocks and adakitic intrusions.Here wepresent a systematic study on zircon U-Pb dating and Lu-Hf isotopes,and whole-rock Sr-Nd-Pb-Os and Mg-O isotopes of the Tibetan ultrapotassic rocks?including the entrained peridotite xenoliths and crustal xenoliths?and coeval potassic volcanic rocks with intermediate to felsic compositions.We have investigated zircon xenocrysts from mantle-derived ultrapotassic rocks and found that they have similar zircon U-Pbage and Hf isotopic distributions with detrital zircon spectra and surface rocks outcropped in the Lhasa terrane,which demonstrates the continental origin of xenocrystic zircons with high U/Yb and the occurrence of crustal contamination during the evolution of ultrapotassic magma.The increasing DyN/YbN and U/Yb ratios since ⁵5 Ma were interpreted as reflecting significant and progressive crustal thickening in response to India-Asia convergence,suggesting the potential of xenocrystic zircons entrained by mantle-derived rocks to probe into the evolution of overlying crust.The Os-Sr-Mg isotopic data further confirm that the Tibetan ultrapotassic rocks are of mantle origin,but underwent crustal contamination and thus have variably high 187Os/188 Os that obviously deviates from normal mantle reservoir.Still some samples with low187Os/188 Os exhibit inverse ?²⁶Mg-87Sr/86 Sr correlations and positive co-variations between ?²⁶Mg and Hf/Sm,suggesting the potential of recycled dolomites to modify mantle Mg isotopic composition.The correlated spatial variations of ?²⁶Mg and Hf/Sm were interpreted as resulting from carbonatitic metasomatism during the northward subduction of Neo-Tethyan oceanic slab.Although postcollisional potassic volcanic rocks and adakitic intrusions in southern Tibet exhibit “adakitic geochemical signatures”,these two rock types have contrasting temporal-spatial distributions and isotopic signatures.We found that adakitic intrusions are concentrated in southern Lhasa subterrane,and derived from partial melting of overthickened lower continental crust.The unradiogenic ¹⁴³Nd/¹⁴⁴ Nd and the dramatic decrease of co-magmatic zircon ?Hf?t?values since ³5 Ma shown by postcollisional adakites should be interpreted as documenting the Indian crustinput.On the other hand,potassic volcanic rocks share similar spatial distributions with ultrapotassic rocks,and their whole-rock Pb and zircon Hf isotopic signaturesindicatetheir affinity with Lhasa terrane and very limited input from underthrusted Indian crust.Aremarkable feature for the early-Miocene potassic volcanic rocks is their bimodal distribution with overlying ultrapotassic lava,and the isotopic discrepancies between these two rock types indicatelimited contribution from ultrapotassic magma.This isotopic discrepancy is diminishing with postcollisional volcanic activity become younger and migrate eastward,suggesting that potassic volcanism with the younger ages may have undergone more extensive hybridization with the associated ultrapotassic melts.Because of being nearly coeval with important geologic episodes in early-Miocene,including the onset of accelerated plateau uplift,high rates of denudation and sedimentation,rapid increase of the 87Sr/86 Sr in seawater,and changes in crustal stress state and deformation style,the postcollisional magmatism in southern Tibet was interpreted as products of convective thinning or delamination of overthickened mantle lithosphere.